Method for Producing Metal Sheets from a Magnesium Melt

ABSTRACT

A method for producing metal sheets from a magnesium melt, comprises the following steps: producing the magnesium melt, casting the magnesium melt to a cast strip, immediately after casting, strip-rolling the cast strip to a rolled strip, cross-cutting the rolled strip to metal sheets, rolling the metal sheets to a final thickness, the rolling of metal sheets being carried out, in relation to the metal sheets, in a direction a at right angle to the rolling direction of the strip-rolling, said direction being maintained during the entire rolling of the metal sheets. The method according to the invention enables very wide sheets to be produced in a simple way which meet the end user&#39;s requirements.

The invention relates to a method for producing metal sheets from a magnesium melt.

Magnesium flat products are increasingly used in the field of car body construction and comparable application areas for the production of particularly light and at the same time inherently stable component parts.

So-called “continuous casting and rolling” represents an advantageous way of producing strip with a certain thickness, wherein a metal melt is cast to a strip in a thickness close to final dimension and directly afterwards is hot-rolled inline to a hot-rolled strip. The resulting cast and hot-rolled strip is then processed further. In this case, other rolling steps or forming operations as well as annealing treatments may be included (DE 100 52 423 C1).

The width of the strip produced in this way is limited by the width of the roll gap of the roll stand used for its forming. Only so-called “slit strip”, the width of which is less than the width of the original product, can be separated from this strip by lengthwise cutting. However, on the part of the users of such flat products the demand not only exists for a wide spectrum of the most varied thicknesses, but there is also a requirement for widths adapted to the respective intended purpose, which exceed the widths which can be produced on conventional mill trains.

Based on the prior art explained above, the object of the invention consisted of indicating a method, which in a simple way enables magnesium metal sheets of greater width and good mechanical properties to be produced.

This object has been achieved by a method of the kind described initially, which for the production of magnesium metal sheets comprises the following steps:

-   -   producing the magnesium melt,     -   casting the magnesium melt to a cast strip,     -   immediately after casting, strip-rolling the cast strip to a         rolled strip,     -   cross-cutting the rolled strip to metal sheets,     -   rolling the metal sheets to a final thickness, the rolling of         metal sheets being carried out, in relation to the metal sheets,         in a transverse direction to the rolling direction of the         strip-rolling, said direction being maintained during the entire         rolling of the metal sheets.

In accordance with the invention, firstly a cast strip, which possesses a certain width limited by the machinery used for rolling, is produced from a magnesium melt in the presently known way. Subsequently, sheets are separated from this strip. These metal sheets are then rolled at right angle to the longitudinal direction of the strip-rolling, if necessary in several passes for such time, until on the one hand the desired width and on the other hand the required thickness are reached. “Rolling direction of the metal sheets” here always means the direction, in which the metal sheet enters the next rolling pass. That is to say, the metal sheet still aligned in the longitudinal direction of the strip immediately after being separated from the cast and rolled strip, is turned for example about an axis perpendicular to the metal sheet surface, so that its direction originally aligned equal to the longitudinal direction of the strip is now aligned at an angle to this. In this way, very wide sheets that meet the requirements of the end user can be produced with a technically particularly simple process.

It is clear that metal sheets produced according to the invention have good mechanical properties and a structure, which is advantageous for their subsequent processing. In this case, the deviations between the properties determined in the longitudinal direction, cross-direction and diagonal direction of the metal sheets produced according to the invention are small. Apart from the advantage of being able to produce a wide metal sheet the positive effects, which are already described in the technical literature for rolling magnesium metal sheets, using repeated directional changes, (Beck, Adolf “Magnesium and its alloys”, Springer, 2nd edition 2001, chapter: Technology of rolling) already arise with the process steps according to the invention. It is surprising in this case that the resulting characteristic improvements occur with the rolling method according to the invention, although the metal sheets separated from the strip are only moved once, so that their alignment during rolling deviates from the alignment, with which the strip has been rolled.

In order to achieve particularly good ductility and uniform distribution of the good mechanical properties of metal sheets produced according to the invention, the inventive method should be carried out so that the metal sheets obtained have little segregation. For this purpose, it may be advantageous to heat the metal sheets to an initial rolling temperature in particular above 200° C. if the temperature of the metal sheets is below 200° C. Typical initial rolling temperatures in this case lie within a range of 200° C. to 450° C.

Rolling the metal sheets may be carried out if necessary in several rolling passes, in order to produce particularly thin, wide metal sheets. If multi-pass rolling is used, it is advantageous if the metal sheets are heated to an initial rolling temperature between each rolling step in the presently known way.

The invention is described below in detail on the basis of a drawing illustrating an exemplary embodiment, wherein there are shown:

FIG. 1 a production line for producing wide metal sheets from a magnesium melt in schematic illustration from above;

Diag. 1 the yield point R_(p0.2) and tensile strength recorded in each case in longitudinal direction, cross-direction and diagonal direction for a magnesium strip cold-rolled lengthwise in the conventional way and a magnesium strip cold-rolled at right angle in the way according to the invention.

The production line 1 comprises a smelting vessel 2 from which magnesium melt S is continuously fed by means of a nozzle 3 into the casting slit, not illustrated, of a two-roller strip-casting machine 4.

The strip-casting machine 4 in the presently known way has two casting rollers arranged vertically one above the other, not illustrated here, which between them limit the horizontally running casting slit.

The cast magnesium strip M leaving the strip-casting machine 4 is continuously conveyed in an operation following the strip-casting into a rolling stand 5 positioned inline with the strip-casting machine 4, in which it is rolled to a hot-rolled strip W of for example 6.5 mm in thickness. From the hot-rolled strip W leaving the roll stand 5, metal sheets B are separated by means of cross-cutting shears 6 arranged inline in the conveying direction F behind the roll stand 5. If necessary, before the cross-cutting shears 6 the hot-rolled strip W is trimmed along its sides by a trimming machine, not illustrated.

The metal sheets B separated from the hot-rolled strip W have a rectangular shape when viewed from above the longitudinal sides LS of which aligned in the longitudinal direction L of the magnesium strip M are narrower than their transverse sides QS aligned at right angle to the longitudinal direction L of the hot-rolled strip W. Their width is equal to the width b of the hot-rolled strip W.

From the cross-cutting shears 6, the metal sheets B reach a conveyor section 7, at the end of which they are picked up by the swivel arm 8 of a stacker device 9. The swivel arm 8 is pivotably arranged about a vertical axis 10 so that it rotates about the vertical axis 10 with a 90° turn. The metal sheet B picked up in each case thereby rotates 90° about an axis 11 normally aligned with the top side of the metal sheet B.

The stacker device 9 places the metal sheets B on a stack 12. In this position the long sides LS of the metal sheets B are aligned at right angle to the longitudinal direction L of the hot-rolled strip W.

A conveyor device 13 in each case takes one of the stacked metal sheets B to a cold-rolling mill 14 so that firstly with its one short longitudinal side LS1 it enters the roll gap, not illustrated, of the cold-rolling mill 14. The respective metal sheet B in this way is rolled in a rolling direction Q, which is aligned at right angle, that is to say offset 900, to the longitudinal direction L with which the hot-rolled strip W has been originally rolled. By 6-fold cold-rolling with intermediate heating in a furnace 15 a 0.9 mm thick metal sheet Bf, the length LBf of which is substantially greater than the width B of the hot-rolled strip W, is finish-rolled from the 6.5 mm thick metal sheet B in each case.

The resulting metal sheets Bf are finally annealed at 330-360° C.

The finished, finally-annealed metal sheets Bf have good mechanical properties. Thus, in the case of the magnesium metal sheets Bf examined here the yield point R_(p0.2) were in the range of 165-170 N/mm², the tensile strength R_(m) in the range of 250-270 N/mm² and the elongations A₅₀ in the range of 15-20%.

Diagram 1 compares the R_(m), R_(p0.2) and A₅₀ values for lengthwise- and cross-rolled metal sheets, wherein the values illustrated as lines represent the lengthwise-rolled material and the rectangles, dots and stars represent the measured values for cross-rolled material.

REFERENCE SYMBOLS

-   1 production line -   2 smelting vessel -   3 nozzle -   4 two-roller strip-casting machine -   5 roll stand -   6 cross-cutting shears -   7 conveyor section -   8 swivel arm -   9 stacker device -   10 vertical axis of the swivel arm -   11 axis about which the metal sheets B are turned -   12 metal sheet stack -   13 conveyor device -   14 cold-rolling mill -   15 furnace -   B metal sheets -   Bf finish-rolled metal sheet -   b width of the hot-rolled strip W -   F conveyor direction -   L longitudinal direction of the magnesium strip M -   LBf length of the finish-rolled metal sheet Bf -   LS longitudinal sides of the metal sheets B -   LS1 longitudinal side, with which the respective metal sheet B     enters the cold-rolling mill     -   M magnesium strip     -   QS transverse sides of the metal sheets B     -   S magnesium melt     -   W hot-rolled strip 

1. A method for producing metal sheets from a magnesium melt comprises the following steps: producing the magnesium melt, casting the magnesium melt to a cast strip, immediately after casting, strip-rolling the cast strip to a rolled strip, cross-cutting the rolled strip to metal sheets, rolling the metal sheets to a final thickness, the rolling of metal sheets being carried out, in relation to the metal sheets, in a direction at a right angle to the rolling direction of the strip-rolling, said direction being maintained during the entire rolling of the metal sheets.
 2. Method according to claim 1, wherein rolling of the metal sheets is carried out in several rolling passes.
 3. Method according to claim 1, wherein the metal sheets are heated to an initial rolling temperature, if their temperature is below 200° C.
 4. Method according to claim 2, wherein the metal sheets are heated to an initial rolling temperature between each rolling step.
 5. Method according to claim 1, wherein the finish-rolled metal sheets are subjected to final annealing at 330° C.-360° C. 